In a discharge pumping type gas laser device, some atmospheric air seeps into the blast system when the device is not operating, and therefore, after starting, the laser gas must be displaced by not less than a predetermined amount of fresh gas. If this operation is not performed, usually the control characteristic of the laser output is adversely affected. More specifically, the first transition of an actual laser output for a step command value is delayed.
Accordingly, the laser gas is displaced by this predetermined amount, which is kept at a constant value from the start to the end of the operation.
If the operation is continued for a predetermined period, however, most unnecessary components introduced therein during the non-operating state are exhausted to the outside, so that the control characteristic of the laser output cannot be lowered without displacing the amount required immediately after the start, and thus it is pointless to keep the displacement amount at a constant value during the period from the start to the end of operation.